This invention relates to titanium alloys, particularly to dispersion strengthened titanium alloys.
The high strength-to-density ratio of titanium makes it a very attractive design choice in energy-efficient high thrust-to-weight gas turbine engines or airframes of modern airplanes. In titanium, the alloying elements tend to stabilize either the low-temperature close-packed hexagonal alpha phase, or the higher temperature allotrope, body-centered cubic beta phase. Titanium alloys for aerospace applications generally contain both alpha and beta stabilizing elements in various proportions depending on the application and, therefore, the required mechanical properties. The variety of compositions in titanium alloys arises in part because certain alloys are designed for optimization of certain properties. For example, for short-term strength, a relatively high beta stabilizer content is required, while for long-term creep strength, a relatively higher alpha stabilizer content is required.
The important high-temperature properties for aerospace related applications of titanium alloys are: tensile strength, creep, fatigue initiation and fatigue crack propagation resistance, fracture toughness, hot salt stress and corrosion cracking, and oxidation resistance. In addition to selection of an alloy composition, processing of an alloy can be employed to provide desired properties.
In near-alpha and alpha+beta titanium alloys, the creep strength may be substantially increased by heat treating or processing the material above the beta transus temperature to obtain large beta grain size and a transformed beta lenticular alpha morphology.
The creep resistance of titanium alloys can also be improved by dispersion strengthening alloying additions, such as metalloids or rare earth oxides or oxysulfides, to the alloy matrix. Such additions form second phase particles which, if spherical, if small enough, and if uniformly distributed throughout the matrix, provide barriers which prevent dislocation movement such that the resistance of the material to high temperature deformation, and hence the high temperature strength of the material, is increased. In order to provide a stable dispersion to prevent movement of dislocations, the precipitates must be fine, i.e., on the order of less than about 1000 Angstroms in diameter, uniformly dispersed throughout the matrix, spherical in structure, and of relatively high volume fraction, i.e., about 5% or greater. However, dispersoids tend to coarsen with increasing temperature, so that ultimately they become ineffective for creep resistance if the material is exposed to high temperature during processing or during service.
Therefore, in near alpha and alpha+beta titanium alloys containing dispersoids, it is difficult to obtain a large beta grain size, a transformed beta lenticular alpha morphology, and a fine dispersoid structure, the desirable combined microstructural characteristics for high temperature creep resistance, at the same time, by conventional means. While high temperature treatment or processing may result in desired beta and alpha grain structure, the material will develop an undesirably coarse dispersoid structure.
It is an object of the present invention to provide a method to produce titanium alloy articles having high creep resistance.
Other objects and advantages of the present invention will be apparent to those skilled in the art from the following description of the invention.